The present invention relates to reactors for the plasma-assisted processing of gaseous media and in particular to such reactors for the reduction of the emission of carbonaceous and nitrogenous oxide combustion products from the exhausts of internal combustion engines.
One of the major problems associated with the development and use of internal combustion engines is the noxious exhaust emissions from such engines. Two of the most undesirable materials, particularly in the case of diesel engines, are particulate matter (primarily carbon) and oxides of nitrogen (NOx). Increasingly severe emission control regulations are forcing internal combustion engine and vehicle manufacturers to find more efficient ways of removing these materials in particular from internal combustion engine exhaust emissions. Unfortunately, in practice, it is found that combustion modification techniques which improve the situation in relation to one of the above components of internal combustion engine exhaust emissions tend to worsen the situation in relation to the other. A variety of systems for trapping particulate emissions from internal combustion engine exhausts have been investigated, particularly in relation to making such particulate emission traps capable of being regenerated when they have become saturated with particulate material.
Examples of such diesel exhaust particulate filters are to be found in European patent applications EP 0 010 384; U.S. Pat. Nos. 4,505,107; 4,485,622; 4,427,418; and 4,276,066; EP 0 244 061; EP 0 112 634 and EP 0 132 166.
In all the above cases, the particulate matter is removed from diesel exhaust gases by a simple, physical trapping of particulate matter in the interstices of a porous, usually ceramic, filter body, which is then regenerated by heating the filter body to a temperature at which the trapped diesel exhaust particulates are burnt off. In most cases the filter body is monolithic, although EP 0 010 384 does mention the use of ceramic beads, wire meshes or metal screens as well. U.S. Pat. No. 4,427,418 discloses the use of ceramic coated wire or ceramic fibres.
In a broader context, the precipitation of charged particulate matter by electrostatic forces also is known. However, in this case, precipitation usually takes place upon large planar electrodes or metal screens.
GB patent 2,274,412 discloses a method and apparatus for removing particulate and other pollutants from internal combustion engine exhaust gases, in which the exhaust gases are passed through a bed of charged pellets of material, preferably ferroelectric, having high dielectric constant. In addition to removing particulates by oxidation, especially electrical discharge assisted oxidation, there is disclosed the reduction of NOx gases to nitrogen, by the use of pellets adapted to catalyse the NOx reduction.
A problem which arises with plasma assisted gas processing reactors which include a bed of pellets of a high-dielectric constant material, such as those exemplified in specification GB 2 274 412, is that localised variations in the electric field in the pellet bed can occur, possibly leading to regions of the pellet bed in which the electric field is insufficient to enable a plasma to be established in a gaseous medium flowing through the pellet bed of the reactor.
U.S. Pat. No. 5,746,051 discloses a plasma reactor of the silent discharge type in which an array of flat plate electrodes is interleaved with flat dielectric plates. Such an arrangement, however, limits the space both for gas flow and also for containment of catalytic packing, for example. Also, such an arrangement restricts gas flow to the essentially two dimensional regions between the plates.
U.S. Pat. No. 5,855,855 discloses a corona discharge reactor in the form of a dielectric material tube with a wire inside and a distributed electrode outside the dielectric tube. The active space within which the corona discharge occurs is that contained within the dielectric tube. An assembly of a plurality of such reactors in parallel is also disclosed.
It is an object of the present invention to provide an improved reactor for the plasma-assisted processing of gaseous media.
According to the invention there is provided a reactor for the plasma-assisted processing of gaseous media, comprising a reactor chamber including a gas permeable bed of an active material, means for constraining a gaseous medium to flow through the bed of active material, wherein the bed of active material comprises a matrix array of components of dielectric material, characterised in that the matrix array comprises a plurality of first wire or rod form electrically conducting members interspersed with a plurality of second wire or rod form electrically conducting members, the first and second electrically conducting members being in electrical contact with and enveloped by dielectric material, the plurality of first electrically conducting members being connected together for connection to a first electrical supply terminal and the plurality of second electrically conducting members being connected together for connection to a second electrical supply terminal. In use a power supply is connected to apply an electrical potential across the first and second electrical supply terminals, the potential and the arrangement of the array being such as to establish a plasma in a gaseous medium flowing through the bed of active material. In practice, it will be appreciated, one of the electrical supply terminals will be connected to earth whilst the other is connected to a high voltage input supply.
The dielectric constant or permittivity of the said dielectric material is selected so as to optimise the plasma-assisted processing of the gaseous media flowing through the bed. An intimate contact between the electrically conducting members and their associated dielectric material is preferred to avoid plasma formation in any voids therebetween. The surface of the dielectric material in contact with the associated electrically conducting member may be coated with a metallic or other conducting coating to optimise this contact and prevent plasma formation therebetween and thus increase the electrical efficiency of the reactor for processing of the gaseous media.
In one embodiment of the invention the said dielectric material is in the form of beads. Any one or a mixture of a variety of shapes may be adopted for the beads. The dielectric strength of the bead material is important in determining the size of bead that can be used in order to avoid electrical breakdown through the body of the dielectric. The higher the dielectric strength the smaller the bead size which can be used.
In another embodiment a dielectric coating is applied on electrically conducting members in the form of a wire or rod, the coating thereby forming the component of dielectric material in situ. Such a coating may be deposited by a variety of methods including thermal spraying, for example by plasma-spraying as well as by wet chemical techniques for example by sol-gel processing. Dielectric material in the form of beads or coating can have catalytic properties for processing of gaseous media. This may be particularly useful when for example processing nitrogeneous oxides and hydrocarbons in internal combustion engines as described in our publication WO99/12638 and the specification of our application PCT/GB00/00079. Reductant gases such as hydrocarbon or nitrogen-containing reductant as described in PCT/GB00/00079 can be introduced before, after or into the reactor for processing of gaseous media. The reactor may be incorporated in a complete emission control system, in which emission control system catalyst material and reductant gases are utilised separately from the reactor.
Preferably, the matrix array comprises first and second electrically conducting members enveloped by dielectric material arranged in alternate rows.
Examples of materials for the dielectric material include the aluminas known as LD 350, CT 530, Condea hollow extrudates, DYPAC, T-60 Alumina, T-162 alumina cordierite, xcex1, "khgr" and xcex3 aluminas, and aluminas containing mixtures of these phases; ferroelectric materials such as titanates, particularly barium titanate; titania, particularly in the anatase phase; zirconia, vanadia, silver aluminate, perovskites, spinels, metal-doped zeolites and mixtures of these compounds. Examples of zeolites are those known as ZSM-5, Y, beta, mordenite all of which may contain iron, cobalt or copper with or without additional catalyst promoting cations such as cerium and lanthanum. Other examples of zeolites are alkali metal containing zeolites such as sodium Y zeolites. Examples of perovskites are La2CuO4, La1.9K0.1CU0.95 V0.05 O4 and La0.9 K0.1CoO3. Vanadates including metavanadates and pyrovanadates such as potassium metavanadate, caesium metavanadate, potassium pyrovanadate and caesium pyrovanadate are also examples of dielectric materials. Selection of the permittivity is a parameter for optimisation of the plasma process. For example low permittivity material such as aluminium oxide or zeolite can be used for some plasma processing and higher permittivity material such as barium titanate for others.